In 1994, the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) introduced a new typology of Attention Deficit/Hyperactivity Disorder (AD/HD), which recognizes Predominantly Inattentive (IN) and Predominantly Hyperactive- Impulsive (HI) subtypes of the disorder, in addition to the Combined type (CB), which encompasses both sets of symptoms and corresponds to the traditionally recognized disorder. Population-based studies, as well as the DSM-IV field trials, have shown that the IN subtype is common, accounting for more than 25 percent of clinic-referred children diagnosed with AD/HD. Furthermore, these studies have documented that children with the IN subtype are as academically impaired as those with the CB type, and suffer significant behavioral and social impairments as well. Behavioral profiles of the two types differ markedly; whereas the CB subtype is characterized by disinhibition, the IN type is lethargic, socially withdrawn, and slower in cognitive tempo. Although these observations point to significant differences in the underlying neurobiology of the two subtypes, virtually no research has compared the IN and CB subtypes with respective to cognitive function, neurophysiology, etiology, family psychiatric history, clinical course, or response to treatment. As the first in a planned series of studies in these areas, the specific aim of the current proposal is to compare the neurocognitive functioning of IN, CB and normal control groups with respect to attention (orienting, selective, focused, and sustained), learning (simple and complex), memory (working, short- and long-term), inhibitory control, and executive functions. These measures have been selected to maximize the likelihood of identifying the neuropsychological processes (eg. arousal vs activation) and particular brain regions (eg. dorsolateral vs. orbital prefrontal cortex), which may mediate any deficits identified. Each group will enroll thirty male and female children, 7-10 years of age, matched across groups for age and sex, and balanced for minority and poverty status.